Such mass beach robberies were once a hair-raisingly frequent occurrence, but they had largely disappeared in recent years as this notoriously dangerous city got markedly safer — a trend credited to a galloping economy and police operations that wrested control of more than 200 favela hillside slums from the drug-dealing gangs that controlled them.

However, the arrastoes, or “big drags” as they are known in Portuguese, are back, and the gang raids on Nov. 15 and Wednesday last week spread alarm through a city gearing up to host soccer’s World Cup in just over six months and the Summer Olympics in 2016.

“What’s happening in Rio today represents a power play,” wrote Merval Pereira, a columnist for the Rio newspaper O Globo. “Since the police’s pacification program was put into place, the bandits have been losing control over large swaths of the territory in which they used to act and are looking to take back what was theirs.”

Police initially dismissed the incidents as stampedes caused by fights, but later acknowledged they were mass robberies. Officials announced they will step up weekend beach patrols and set up mobile police posts to make it easier for victims to report crimes.

Local newspapers have reported that 15 people, most of them minors, were detained following Wednesday’s incident on Arpoador Beach, which saw repeated stampedes as swarms of young people swooped down on bathers and the police gave chase. Globo television network broadcast images of officers chasing shirtless youths across streets and stones being hurled at officers as they processed those detained.

“People were running all over the place, and I didn’t know where to go for safety,” said Luana Santos, a 24-year-old vendor of bottled water. “I was really panicked and really frightened. I hope this is not going to keep on happening.”

Rio State Security Secretary Jose Beltrame told CBN radio these were the first instances of mass beachside robberies in Rio in seven years and said two of those detained on Wednesday were minors.

Ezequiel Soliva de Andrade, a 39-year-old waiter at a bar on the boardwalk on Arpoador, said he holds little hope the extra policing will do much to stop the crime.

“Every time the sun comes out, there are tonnes of robberies,” Andrade said, adding that the problem has gotten much worse over the past three months. “These guys don’t care whether there are cops there are not. They just take off running and there are so many of them going in all different directions that they’re rarely caught.”

Andrade rattled off a long list of muggings and other attacks on beachgoers and neighborhood residents that he has witnessed over the past few months. He said the crime wave is scaring people away.

“We used to sell more than 900 coconut waters a day,” he said. “Now we barely sell 200.”

Rio is attempting to burnish an image tarnished by violence and police brutality during mass protests earlier this year. About 500,000 foreigners are expected to flood into Brazil for next year’s World Cup, and authorities are wary that reports of crime could scare some big-spending visitors away.